A device for keyless locking and unlocking of a door is described in "Keyless Entry System with Radio Card Transponder," Motoki Hirano et al., IEEE Transactions on Industrial Electronics, vol. 35, 1988, pages 208 through 216. A door with an electronic lock can be locked and unlocked with this known device because a transmitter unit mounted on a motor vehicle queries a response unit (transponder) assigned to the user and causes the electronic lock to be unlocked when the authorized user is identified. One problem encountered with such a device for locking and unlocking is that an additional battery must be provided to permit operation of the device with the electronic lock in the event the vehicle battery is dead.